.toUpperCase or .toLowerCase but specify Locale/ENGLISH.
Wednesday, April 29, 2009
.toUpperCase
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I recently learnt that when you want to convert the case of a technical identifier (a tagname, a HTTP header etc.) you must not use plain
Tuesday, April 28, 2009
Counting occurences (solution to the exercise)
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- What does
(merge-with + {:a 12} {:b 4} {:a 3 :b 7})return? {:b 11, :a 15}when there are several values for a key, these values are merged (two at a time) using the specified function — here they are summed.- Can you count occurrences of each value in a collection using
merge-with? (apply merge-with + (map (fn [x] {x 1}) coll))or, usingfor:(apply merge-with + (for [x coll] {x 1})).
Monday, April 27, 2009
Screenscraping with Enlive
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(select (html-resource (java.net.URL. "http://clojure-log.n01se.net/")) [:#main [:a (attr? :href)]]) returns a seq of link nodes.
Counting occurences
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One asked me how to count occurrences of each value in a collection. I answered
Exercise:
(reduce #(assoc %1 %2 (inc (%1 %2 0))) {} coll). Since it can take some time to get accustomed to the functional way of thought, here is how one can work such an expression out:- How to count all occurrences of 42?
(count (filter #{42} coll))- How to express count using
reduce? (defn my-count [coll] (reduce (fn [n _] (inc n)) 0 coll))- How to count all occurrences of 42 using
reduce? (reduce (fn [n _] (inc n)) 0 (filter #{42} coll))- Can you get rid of the
filter? (reduce (fn [n x] (if (= 42 x) (inc n) n)) 0 coll)- I'd like the result to be
{42 occurences-count}. (reduce (fn [m x] (if (= 42 x) (assoc m 42 (inc (m 42))) m)) {42 0} coll)- 42 is hardcoded in four places, it's bad!
(reduce (fn [m x] (if (= 42 x) (assoc m x (inc (m x))) m)) {42 0} coll)- Can't you replace
{42 0}with{}? - No
(inc (m x))would fail because(m x)would returnnil. - How does one provide a default value when the key is not found ?
(a-map a-key default-value)- Can't you replace
{42 0}with{}? (reduce (fn [m x] (if (= 42 x) (assoc m x (inc (m x 0))) m)) {} coll)- I changed my mind I'd like you to count occurrences of each value.
- Easy!
(reduce (fn [m x] (assoc m x (inc (m x 0)))) {} coll)or, terser,(reduce #(assoc %1 %2 (inc (%1 %2 0))) {} coll)
Exercise:
- What does
(merge-with + {:a 12} {:b 4} {:a 3 :b 7})return? - Can you count occurrences of each value in a collection using
merge-with?
Wednesday, April 22, 2009
Vicious bug
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- What's wrong with this code
(do (defmacro foobar [x] (doto x println)) (foobar (+ 1 1)))? - Nothing.
- Run it again.
- The first time,
foobaris treated as a function (its argument has been evaluated) becausefoobaris unknown before the whole top-level expression is compiled.
The second time, the var namedfoobarpreexists and is a macro,(foobar (+ 1 1))is expanded.
with-test to test macros. It's the kind of bug that goes unnoticed while developing incrementally and evaluating in the same REPL, it was waiting for me to start a fresh REPL.
Sanitizing HTML with Enlive
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net.cgrand.enlive-html=> (sniptest "<div id=user-data>"You also need to remove most attributes but it's just a demo of something that was impossible with the old Enlive.
[:#user-data] (html-content "code injection<script>alert('boo')</script>")
[:#user-data (but #{:p :br :a :strong :em})] nil)
"<html><body><div id=\"user-data\">code injection</div></body></html>"
By the way, the old Enlive is no more. Long live the new Enlive!
Saturday, April 18, 2009
Make it work, make it right, make it fast
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Make it work, make it right, make it fast.
See the README file to know what's new and here for an example.
- Make it work
- sort of done: the old Enlive,
- Make it right
- work in progress, the new Enlive,
- Make it fast
- once the "right" branch is merged into master.
See the README file to know what's new and here for an example.
Friday, April 17, 2009
Mapping every second item
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I wanted to apply a function to every second item in a coll. I was considering writing something using
interleave, take-nth and map or a combination of mapcat and partition when I thought of this:(map #(%1 %2) (cycle [f identity]) coll)I really love clojure's map parallel processing. (I should ask if
every? and some could be allowed to take several colls.)
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